1. Field of the Invention
The present invention relates to the mixer of a direct conversion receiver, and more particularly to a second intercept point (IP2) calibrator that calibrates the IP2 of a direct conversion receiver's mixer and a method of calibrating the IP2.
2. Description of the Related Art
When radio frequency input signals having two or more input frequencies pass through non-linear systems or non-linear circuits, undesired output frequencies that are different from the input frequencies are caused by non-linearity characteristics of the systems or circuits. This phenomenon is referred to as Intermodulation Distortion (IMD). Intermodulation Distortion (IMD) represents distortion caused by “Inter-modulation” (IM) components. The IM components have frequencies corresponding to the sum of the two input frequencies and the difference between the two input frequencies. Thus, when the input signals having two different input frequencies are applied to the non-linear systems or non-linear circuits, the IMD causes interference with modulation and demodulation.
A theoretical point where a linear extension of the second order IMD intersects a linear extension of an input signal is referred to as second intercept point (IP2). The IP2 is an important parameter used to characterize the linearity of a communication system. As a power level of the input signal increases, the power level of the second order IMD also increases, and the point where the power level of the second order IMD intercepts the original power level of the input signal represents the IP2. However, since output power is saturated before the output power reaches theoretical IP2, the real IP2 corresponds to an expected hypothetical output power level where the second order IMD is expected to reach the same amplitude level as the input power level.
A third intercept point (IP3) is significant in the case of communication employing a superheterodyne architecture using an intermediate frequency (IF). A superheterodyne transmitter converts base-band signals into IF signals and converts the IF signals into radio frequency (RF) signals to transmit the RF signals. A superheterodyne receiver converts received RF signals into IF signals and converts the IF signals into base-band signals.
On the other hand, the second intercept point (IP2) is significant in the case of communication employing a direct conversion architecture that does not use IF. A direct conversion transmitter directly converts base-band signals into RF signals to transmit the RF signals. A direct conversion receiver directly converts received RF signals into base-band signals. Because second order IMD occurs at base-band frequencies, the second order IMD causes greater signal distortion than third order IMD. Accordingly, in the direct conversion architecture, there is a need for adjusting the second order IMD to prevent the signal distortion. The linearity of the communication system may increase by achieving high IP2, which reduces the second order IMD.
Generally, a mixer in the direct conversion receiver has an IP2 calibration circuit for adjusting the IP2.
FIG. 7 is a circuit diagram of a conventional IP2 calibration circuit.
Referring to FIG. 7, the conventional IP2 calibration circuit includes a mixer 10 and an IP2 controller 20. The mixer 10 includes a first pair of input terminals 2 for receiving an RF signal Vrf and a second pair of input terminals 4 for receiving a local oscillation signal Vlo corresponding to the known carrier frequency of the RF signal Vrf. The mixer 10 outputs the base-band signal having a frequency equal to the difference between the frequency of the RF signal Vrf and the frequency of the local oscillation signal Vlo. The base-band signal is outputted at a pair of output terminals 6.
The IP2 controller 20 includes load resistors RLP and RLN, and a calibration resistor Rcal. The calibration resistor Rcal is connected in parallel to the load resistor RLP as shown in FIG. 7 (or in parallel to RLN, not shown). The calibration resistor Rcal compensates for any mismatch between differential outputs Vop and Von of the mixer 10.
A total second order intermodulation (IM2) output voltage is obtained by summing up the IM2 output voltage in common-mode and the IM2 output voltage in differential-mode.
The IM2 output voltage VIM2,cm in common-mode is given by following Expression 1.VIM2,cm=icm(R+ΔR−Rc)−icm(R−ΔR)=icm(2ΔR−Rc),  [Expression 1]
where Rc denotes a decrease in a resistance value of the load resistor RLP due to the calibration resistor Rcal, and icm denotes a current in common-mode.
The IM2 output voltage VIM2,dm in differential-mode is given by following Expression 2.VIM2,dm=idm(R+ΔR−Rc)+idm(R−ΔR)=idm(2R−Rc),  [Expression 2]
where Rc denotes the decrease in the resistance value of the load resistor RLP due to the calibration resistor Rcal, and idm denotes a current in the differential-mode.
Therefore, the total IM2 output voltage VIM2 is given by following Expression 3.VIM2=VIM2,cm+VIM2,dm=Idm(2R−Rc)+icm(2ΔR−Rc)  [Expression 3]
The IP2 is calibrated by adjusting the Rc to reduce the total IM2 output voltage VIM2.
The above-mentioned calibration method has limitations associated with the semiconductor manufacturing process. Since ΔR is in a range of from about 0.1% to 10% of R, Rc is also in a range of from about 0.1% to 10% of R. Additionally, Rcal needs to be ten times to thousand times as large as the resistance of R, thus, when R is tens of KΩ, Rcal needs to be tens of MΩ. Therefore, Rcal is difficult to be implemented in a semiconductor manufacturing process, since a considerably large resistor occupies a large area on a semiconductor substrate. Additionally, the IP2 calibration circuit using a resistive load for IP2 calibration has limitations. For example, a sufficient voltage margin may not be acquired in a structure where a high gain and linearity is required.
For overcoming such limitations, IP2 calibrators using various circuits and methods have been proposed. An IP2 calibration method calibrating a mixer by using a common-mode feedback circuit is disclosed in US Patent Application Publication No. 2006-0145706. An IP2 calibrator using the common-mode feedback circuit may be more easily implemented than the IP2 calibrator in FIG. 1.
However, DC offset may occur when IP2 of the mixer is calibrated by using the common-mode feedback circuit. Further the IP2 characteristic may be degraded when the caused DC offset is removed.